For most horns it is a precision machined, silver plated brass washer. For Bach trumpets it is an altered bottom valve cap.

What it does:

It improves slotting, centers the tone and strengthens the harmonics. This results in a more secure upper register and improved projection.

How it works:

The Reeves cylinder reinforcer prevents the dissipation of energy that often occurs occurs at the bottom of the valve casing. It has the most effect on the 3rd valve as this is where the energy enters the valve section. It has a similiar, yet not as intense effect on the 1st valve (where the energy leaves the valve section), and a negligible effect on the second valve.

This is different than adding mass to the horn by using heavy valve caps.